Monday, October 10, 2011

Spectacle Reef Light, in northeastern Lake Huron near the Straits of Mackinac, was begun in 1870 and first lit in June 1874. The masonry tower is still considered a feat of engineering. How'd they do it?

Funny you should ask...

From “The Light Houses of the United States” by Charles Nordhoff* in Harper’s New Monthly Magazine, March 1874.

The light-house on Spectacle Reef [illustration above], on the coast of Lake Huron, cost $300,000.... The following account of the difficulties encountered in preparing for its construction will give an idea of what natural obstacles have often to be overcome in this kind of building. The account is taken from the official report:

The site of the tower being determined, and the proper soundings and surveys made, a crib ninety-two feet square was built, having a central opening forty-eight feet square to receive the coffer-dam which was to form the pier of protection, as well as the landing-place for materials. This huge crib was floated to its place.

In order to get accurate soundings to guide in shaping the bottom of the crib, and to fix with a degree of certainty the position of these soundings and that to be occupied by the crib, four temporary cribs, each fifteen feet by twenty-five feet, of round timber, were placed in from eight to ten feet of water, in a line corresponding with the proposed eastern face of the pier of protection, and filled to the level of the water with ballast stone. These four cribs were then decked over and connected together. Upon the pier thus formed about seventy cords of ballast stone were placed, ready at the proper time to be thrown into the crib forming the pier of protection.

The lower two complete courses of the pier of protection having been fastened together by screw-bolts, forming a raft, constituting a ground-plan of the pier of protection, were then towed from the harbor where framed to the reef, and moored directly over the position to be occupied by the finished pier. Its position was marked upon the temporary pier referred to above, and soundings taken at intervals of two feet along each timber in the raft, thus obtaining accurate contours of the surface of the reef within the limits of these timbers. The raft was then towed back to the harbor, hauled out upon ways, and by means of wedges of timber the bottom was made to conform to the surface of the reef. The raft, now become the bottom of the pier of protection, was then launched, and additional courses of timber built upon it, until its draught of water was just sufficient to permit its being floated into position on the reef, at which time it was estimated that the top of the pier would be one foot out of water.

The depth of water on the reef at the points to be occupied by the four corners of the pier of protection was found to be as follows: At northeast corner, ten feet six inches; at northwest corner, thirteen feet; at southwest corner, fourteen feet six inches; and at southeast corner, nine feet six inches—the position to he occupied by the pier of protection having been so chosen that the sides would correspond to the cardinal points of the compass. Meanwhile five barges at the harbor had been loaded with ballast stone, making, together with those on the temporary pier at the reef, 290 cords (about 1800 tons) at command, with which to load the pier of protection and secure it to the reef as soon as it should be placed in position.

On the evening of the 18th of July, 1871, every thing being in readiness, and the wind, which had been blowing freshly from the northwest for three days previously, having somewhat moderated, at 8 P.m. the tugs Champion (screw-propeller) and Magnet (side-wheel) took hold of the immense crib and started to tow it to the reef, fifteen miles distant, followed by the Warrington (screw-propeller), having in tow the schooner Belle, the two having on board a working force of 140 men, the tug Stranger (screw-propeller), with barges Ritchie and Emerald, and the tug Hand, with two scows of the Light-house Establishment. The barge Table Rock, with fifty cords of stone on board, was left in reserve at the harbor. The construction scow, with tools, etc., on board, was towed with the crib. At 2 A.M. next morning, six hours after starting, the fleet hove to off the reef, awaiting daylight and the abatement of the wind, which had again freshened up. At 6½ A.M., it having moderated, the pier, with considerable difficulty, was placed in position, and after being secured to the temporary pier and the moorings previously set for the purpose, all hands went to work throwing the ballast stone into the compartments, and by 4 P.M. succeeded in getting into it about 200 cords, or 1200 tons. By this time the wind was blowing freshly, and the sea running so high as to make it necessary to stop work for the time, but early next morning all the reserve stone was put into the compartments.

After the pier was in position the schooner Belle was moored on the reef to serve as quarters for the working force, which proceeded to build up the pier to the required height above water (twelve feet). On the 12th of September the pier had been built up to its full height, and by the 20th of September quarters for the workmen had been completed upon it, which were at once occupied, and the Belle returned to the harbor.

By means of a submarine diver the bedrock within the opening of the pier was then cleared off, and the work of constructing the coffer-dam was taken in hand. The coffer-dam itself consisted of a hollow cylinder, forty-one feet in diameter, composed of wooden staves, each four inches by six, and fifteen feet long. The cylinder was braced and trussed internally, and hooped with iron externally, so as to give it the requisite strength. It was put together at the surface of the water, and when complete was lowered into position on the bed-rock by means of iron screws.As soon as it rested on the rock (which was quite irregular in contour), each stave was driven down so as to fit as closely as it would admit, and a diver filled all openings between its lower end and the rock with Portland cement. A loosely twisted rope of oakum was then pressed close down into the exterior angle between the coffer-dam and rock, and outside of this a larger rope made of hay. The pumping machinery having meanwhile been placed in readiness, the coffer-dam was pumped dry, and on the same day (14th October) a force of stone-cutters descended to the bottom and commenced the work of leveling off the bed-rock, and preparing it to receive the first course of masonry.

The bed-rock was found to consist of dolomitic limestone, confirming the previous examinations, highest on the western side, toward the deepest water, and sloping gradually toward the eastern. In order to make a level bed for the first course of masonry it was necessary to cut down about two feet on the highest side, involving a large amount of hard labor, rendered more difficult by the water forcing its way up through seams in the rock. But the work was finally accomplished, the bed being as carefully cut and leveled as any of the courses of masonry.

The first course of masonry was then set, completing it on the 27th of October. While setting this course much trouble was caused by the water, already referred to as forcing its way up through seams in the rock, which attacked the mortar-bed. For this reason water was let into the dam every evening, and pumped out next morning, to give the mortar time to harden during the night. This mortar was composed of equal parts of Portland cement and screened siliceous sand. Specimens of it obtained the following spring, after being in place under water for seven months, were quite as hard or harder than either the bed-rock or the stone used in building the tower.The weather having now become very boisterous, with frequent snow-squalls, often interrupting the work, and the setting of any additional stone requiring the removal of a portion of the most important of the interior braces of the coffer-dam, it was deemed prudent to close the work for the season. This, too, would give ample time for the hardening of the mortar used in bedding the stone, and the concrete used for filling cavities in the bed-rock, as well as the space between the outside of the first course and the coffer-dam, which was solidly filled with concrete to the top of the first course. Therefore the coffer-dam was allowed to fill with water, the process being hastened by boring holes through it to admit the water, and it was secured to prevent its being lifted by the ice during the winter.

The machinery was laid up, and on the last of October all the working force, except two men, was removed. These two men were left to attend to the fourth-order light which had been established on the top of the men's quarters, and the fog-signal, consisting of a whistle attached to one of the steam-boilers. At the close of navigation they were taken off the pier by the lighthouse tender Haze.

The degree of success of this novel coffer-dam may be inferred from the fact that although prepared with pumps of an aggregate capacity of five thousand gallons per minute, not more than a capacity of seven hundred gallons was used, except when emptying the coffer-dam, and then only to expedite the work. Once emptied, a small proportion of this capacity was ample to keep the coffer-dam free from water; and this at a depth of twelve feet of water, on rock, at a distance of nearly eleven miles from the nearest land. Every person connected with the work may well feel a just pride in its success. All the stone which had been delivered at the harbor, consisting of the first five courses (each course two feet thick), having been cut by this time, the work there was also closed.

The season opened a month later in 1872 than in 1871, consequently work was not resumed at the harbor until the 3d of May, and upon the reef until the 20th of the same month. On the 13th of May the ice in the coffer-dam was still a compact mass, of some feet in thickness. Masses of ice still lay on top of the pier itself. As soon as any thing could be done, the ice still remaining was cleared out of the coffer-dam, the machinery put in order, the braces removed from the interior of the coffer-dam, and then the work of setting additional courses began.

The work upon the tower was carried on at such a rate that one entire course of masonry was set, drilled, and bolted complete every three days.

The Spectacle Reef tower was founded upon a rock the highest part of which was ten feet under water.